In-situ process for detoxifying hexavalent chromium in soil and groundwater

ABSTRACT

Inexpensive, highly effective methods for the in-situ reduction of hexavalent chromium to the non-toxic trivalent oxidation state in soil and groundwater containing hexavalent chromium are provided, which may reduce hexavalent chromium concentration in soil to as low as 5 mg/L. The methods involve sampling soil comprising hexavalent chromium to determine a reaction amount of a reducing agent, providing a frame on a top surface of the soil, wetting the soil, spreading a reducing agent on the top surface of the soil, and flushing the soil with water to dissolve the reducing agent. The reducing agent may be a chemical agent, a biological agent, or a combination of chemical and biological agents. The biological reducing agents may include sludge from wastewater, return activated sludge, waste activated sludge, leachate from landfill or from composting operations, or compost material from municipal wastewater, industrial wastewater, or solid waste operations.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims the benefit of U.S. ProvisionalApplication No. 60/317,786 filed Sep. 6, 2001.

BACKGROUND OF THE INVENTION

[0002] Chromium-containing soils and tailings are significantenvironmental problems in many parts of the United States and around theworld. Chromium compounds are used in metal cleaning, preplating andelectroplating, as well as in the manufacture of inks, paint pigmentsand dyes. The chromium contamination is also a result of chromite oreprocessing, which generated large volumes of tailings and residues thathave high concentrations of hexavalent chromium (Cr(VI)). These tailingsand residues have been deposited as fill materials in many locations inand near the chromium manufacturing sites. As hexavalent chromium is aClass-A human carcinogen through inhalation, detoxification of thesewastes will result in reduced human health risks and allow for futureland use at or near known contaminated sites.

[0003] The United States Environmental Protection Agency (EPA) hasestablished testing criteria for determining acceptable levels ofchromium in the soil. In 1990, it was specifically established as partof the Resource Conservation and Recovery Act (RCRA) that the totalchromium concentration in the leachate of the soil must fall below thestandard Toxicity Characteristic Leaching Procedure (TCLP) limit of 5mg/L in order for the soil to be no longer characterized as “hazardouswaste.” Additionally, the groundwater chromium concentration level isregulated. Currently, the maximum contamination level (MCL) for totalchromium is 100 ppb (parts per billion) or micrograms/liter. TheCalifornia Department of Health Services (CDHS), however, disagrees withEPA on the toxicity of chromium and has set the State MCL to be 50 ppband is proposing to regulate the groundwater hexavalent chromium at 0.2ppb and total chromium at 2.5 ppb. These newer, more stringent levelsare being considered to become effective in 2004 in California.

[0004] Brownfields, or pre-used urban industrial sites, are a type oflocation which may be contaminated with hexavalent chromium. If properlyassessed or cleaned up, such sites could be reused for industrialpurposes, which is desirable over utilizing new clean, suburban sites(greenfields) for similar purposes. Many brownfields are being cleanedup based on the calculated risks associated with various exposurescenarios. Typically, these are based on state laws or regulationsrather than on federal laws or regulations. According to the most recentEPA Region III risk based criteria (RBC) guidelines, which were issuedin April, 2000, the screening concentration for hexavalent chromium is6,100 mg/Kg for industrial sites, 230 mg/Kg for residential areas, and1.5×10⁻⁴ μg/m³ in ambient air. The cleanup targets for brownfields areoften based on these less stringent cleanup standards based onhexavalent chromium concentration.

[0005] The most feasible method for the detoxification of hexavalentchromium is via the well-known reduction to trivalent chromium, Cr(III).Unlike Cr(VI), which is highly soluble, Cr(III) is not a humancarcinogen and is typically found in insoluble forms in the environment.Cr(III) thus represents a lesser health concern than Cr(VI).

[0006] The reactions for the reduction of hexavalent chromium totrivalent chromium in aqueous solution are known. In addition, there arevarying methods in the prior art to attempt to treat and stabilizechromium ore waste, which typically include the use of biological orchemical reduction. Bioremediation processes facilitate the reduction ofCr(VI) to Cr(III) through the use of anaerobic bacteria, whereaschemical reduction methods involve the addition of reducing agents andother reagents to the soil or material to be detoxified.

[0007] Many known processes designed for the reduction of Cr(VI) in soiland other waste materials are known as ex-situ methods, in which thesoil must be excavated and fed through a reactor or apparatus fortreatment. In a typical ex-situ process, such as that disclosed by U.S.Pat. No. 5,304,710, the soil, once excavated, is placed in a reactor andground. The pH of the soil is then adjusted to an appropriate level andcombined with a reduction agent, typically ferrous sulfate, to reducethe hexavalent chromium. Assuming that ferrous sulfate is used as thereducing agent, the following redox reaction applies:

CrO₄ ²⁻+3 Fe²⁻+4 H₂O→Cr³⁺+3 Fe³⁺+8 OH⁻

[0008] Following reduction, further treatments, such as neutralization,may be performed on the soil. The drawbacks generally of ex-situprocessing methods are that large reactors must be constructed and thesoil to be treated must be excavated and transported to the reactor fortreatment, processes which are not efficient on a large scale and can bevery costly and hazardous with respect to the transfer of contaminatedmaterials.

[0009] In-situ methods of soil detoxification are more practical, costeffective and safer, especially when large areas of land must betreated. In this type of approach, one or more reagents are added to thesoil (e.g., in the field) to bring about the reduction. Clear advantagesare the elimination of both the reactor and the need for excavation. Onesuch method is disclosed by U.S. Pat. No. 5,951,457 and involves theaddition of ascorbic acid to the soil to reduce the Cr(VI) to Cr(III).In order to ensure that the chromium in the soil below the ground levelis reduced, the soil must be mechanically mixed with the ascorbic acid.Although this method is designed to treat soil significantly below theground level, extremely large quantities of the acid are necessary. As aresult, the process is not economically feasible on a large scale due tothe high costs of purchasing and transporting large quantities ofascorbic acid.

[0010] Other methods have been proposed for the addition of chemicalreducing agents to the soil. These include (1) first drilling holes inthe ground prior to introducing the reagent; and (2) utilizing arototiller or similar device to thoroughly mix the soil with thereducing agent. One such method, directed toward the reduction ofCr(VI), is described in U.S. Pat. No. 5,285,000. However, deliverymethods designed to inject solutions into soil are typically noteffective methods of delivery because they do not typically provide evendistribution of the reagent to the targeted contaminants. Additionally,the process involves dissolving and mixing ferrous and ferric salts inlarge quantities of water to produce the reducing solutions, which islikely to be quite costly.

[0011] A further such method in U.S. Pat. No. 5,397,478 is directed tothe in-situ reduction of Cr(VI) in soil. This patent demonstrates theuse of hole-drilling only on a very small test plot of soil in alaboratory. It does not provide guidance on how to feasibly implementsuch techniques practically on a large land area, in which the depth ofthe soil is significant, and/or in which large volumes of soil would berequired to be mixed with or otherwise contacted with the reducingagents.

[0012] Bioremediation processes can also be performed in-situ. One suchprocess, described in U.S. Pat. No. 5,681,639, involves stimulating thegrowth of indigenous anaerobic Cr(VI) reducing bacteria in thecontaminated soil and/or groundwater by adding a nutrient medium to thesoil and maintaining a substantially anaerobic environment. Suchnutrients may be carbohydrates, amino acids, organic acids or nitrogensources. However, no reliable means of controlling the biologicalreaction is described.

[0013] Even in view of the above described methods, there remains a needin the art for a method of in-situ soil remediation which is workable,safe, controllable, effective, and economically feasible and which canbe applied on a large scale. There is a further need for anenvironmentally compatible process which is able to achieve soilchromium concentration levels below the TCLP limit and groundwaterchromium concentration below regulatory levels, using inexpensive,easily available reagents and without the need for excavation or mixingof enormous volumes of soil with the applicable chemical reagents.

BRIEF SUMMARY OF THE INVENTION

[0014] This invention relates to a method for the in-situ reduction ofhexavalent chromium to trivalent chromium in soil comprising hexavalentchromium. The method comprises sampling soil comprising hexavalentchromium to determine a reaction amount of a reducing agent, providing aframe on a top surface of the soil, and wetting the soil. The methodfurther comprises spreading a reducing agent on the top surface of thesoil and flushing the soil with water to dissolve the reducing agent.

[0015] This invention also relates to a method for the in-situ reductionof hexavalent chromium to trivalent chromium in soil comprisinghexavalent chromium in a hydraulically isolated area. The methodcomprises sampling soil comprising hexavalent chromium to determine areaction amount of a reducing agent, providing a frame on a top surfaceof the soil, and wetting the soil. The method further comprisesspreading a reducing agent on the top surface of the soil, flushing thesoil with water to dissolve the reducing agent, pumping water comprisingthe reducing agent from an aquifer below the soil and applying thepumped water to the soil.

[0016] This invention also relates to a method for the in-situ reductionof hexavalent chromium to trivalent chromium in soil comprisinghexavalent chromium in a hydraulically isolated area wherein the areacomprises a contaminated waste product selected from the groupconsisting of return activated sludge and waste activated sludge. Themethod comprises sampling soil comprising hexavalent chromium todetermine a reaction amount of a reducing agent, calculating a reductionpower of the contaminated waste product, providing a frame on a topsurface of the soil, and wetting the soil. The method further comprisespumping water comprising the contaminated waste product from an aquiferbelow the soil and applying the pumped water to the soil.

[0017] This invention includes a method for the in-situ reduction ofhexavalent chromium to trivalent chromium in groundwater comprisinghexavalent chromium. The method comprises sampling groundwatercomprising hexavalent chromium to determine a reaction amount of areducing agent, providing a frame on a top surface of the soil andproviding a recovery well and a circulating means on a down gradientside of the frame. The method further comprises wetting the soil,spreading a reducing agent on the top surface of the soil, flushing thesoil with water to dissolve the reducing agent, and pumping watercomprising the reducing agent from the recovery well below the soil andapplying the pumped water to the soil.

[0018] A further method provided by this invention is a method for thein-situ reduction of hexavalent chromium to trivalent chromium in soilcomprising hexavalent chromium. The method comprises sampling soilcomprising hexavalent chromium to determine a reaction amount of areducing agent, providing a frame on a top surface of the soil andproviding a recovery well and a circulating means on a down gradientside of the frame. The method further comprises wetting the soil,spreading a reducing agent on the top surface of the soil, flushing thesoil with water to dissolve the reducing agent, and pumping watercomprising the reducing agent from the recovery well below the soil andapplying the pumped water to the soil.

[0019] A further method provided by this invention is for the in-situreduction of hexavalent chromium to trivalent chromium in soilcomprising hexavalent chromium. The method comprises sampling soilcomprising hexavalent chromium to determine a reaction amount of areducing agent, wetting the soil, spreading a reducing agent on a topsurface of the soil, flushing the soil with water to dissolve thereducing agent, and covering the top surface of the soil with a layer ofclean soil free of hexavalent chromium.

[0020] In addition, this invention includes a system for the in-situreduction of hexavalent chromium in soil comprising hexavalent chromium.The system comprises a frame open at a top and a bottom configured toretain water and a reaction amount of a reducing agent capable ofreducing hexavalent chromium to trivalent chromium, a hose positionedwith respect to the frame such that water can be provided through thetop and bottom of the frame, a spreader capable of delivering a reactionamount of a reducing agent to the frame, and a water source.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

[0021] The foregoing summary, as well as the following detaileddescription of preferred embodiments of the invention, will be betterunderstood when read in conjunction with the appended drawings. For thepurpose of illustrating the invention, there is shown in the drawingsembodiments which are presently preferred. It should be understood,however, that the invention is not limited to the precise arrangementsand instrumentalities shown. In the drawings:

[0022]FIG. 1 is a perspective view of a schematic representation of aframe for use in the invention;

[0023]FIG. 2A is a cross-sectional representative view of a frame andwater source in position on soil for treatment;

[0024]FIG. 2B is an enlarged section of a hose in FIG. 2A; and

[0025]FIG. 3 is a diagrammable view of an embodiment of the method ofthe invention.

DETAILED DESCRIPTION OF THE INVENTION

[0026] The present invention includes methods for the in-situ reductionof hexavalent chromium (Cr(VI)) to trivalent chromium (Cr(III)) in soilincluding Cr(VI). The methods provide for the detoxification of soils,such as tailings, or other similar materials contaminated withhexavalent chromium by reducing the Cr(VI) to the non-toxic Cr(III)oxidation state. As these are in-situ processes, the methods involve thetreatment of the contaminated soil in the field, without the need forexcavating the soil and feeding it through a complicated apparatus orreactor for combination with the reducing agent. Furthermore, a benefitof the methods of the invention is that no mechanical mixing of the soilwith the reducing agent is required. The methods are designed to beapplicable to both small and large areas of land, including those whichare contaminated as a result of chromite ore processing or othermanufacturing processes which utilize chromium. In addition, theinvention provides a method for the in-situ reduction of hexavalentchromium in groundwater which includes Cr(VI). The invention alsoincludes a system for in-situ reduction of Cr(VI) in soil contaminatedwith Cr(VI) as described further below. As used herein, “soil” includessoil, sediment, clay, sand, silt, sludge, residue, tailings and anyother earth-containing or granular material.

[0027] The methods provided by this invention are an environmentallycompatible technology for the in-situ remediation of soils, includingtailings, having penetrability higher than about 1×10⁻⁵ cm/sec. Theprocesses provide ways to reduce chromium concentration in soils,including tailings, to below the TCLP limit of 5 mg/L and to reduce thegroundwater Cr(VI) concentration to below regulatory levels.

[0028] The reduction of Cr(VI) to Cr(III) in soil to be treated may beaccomplished by the use of a chemical reducing agent, a biologicalreducing agent, or a combination of chemical and biological reducingagents, depending on the site conditions. In the first and second cases,the processes are “purely chemical” or “purely biological,” whereas inthe third case, the methods may be characterized as “chemically assistedbiological processes.”

[0029] The chemical reducing agent to be used in the methods may be usedin a variety of forms. Preferred forms include solution, powder, orgranular form. In a preferred embodiment, the reducing agent is a powderor a granulate with an average particle size of less than about 5 mm indiameter and, more preferably, less than about 1 mm in diameter, whereindiameter is measured in the longest dimension of the particulate orgranulate. The reducing agent may be any reducing agent, preferablyeffective in the presence of water, which can effectively reduce Cr(VI)to Cr(III). Preferably, the reducing agent is one or more of a ferroussalt such as ferrous sulfate, ferrous sulfite, ferrous chloride andsimilar materials; a sulfide salt such as sodium sulfide, potassiumsulfide and similar sulfides; sodium thiosulfate; sodium benzoate;and/or an organic reducing agent such as citric acid, methanol, ethanol,formaldehyde, and/or similar materials. The most preferred chemicalreducing agent for the present methods, ferrous sulfate, provideseconomic and environmental advantages. Specifically, ferrous sulfatedoes not contribute to secondary water contamination, and does notrequire regulatory agency approval for its utilization in theenvironment.

[0030] When the process for reducing hexavalent chromium is purelychemical, the overall reaction for the reduction of dichromate ions byferrous species can be represented by the equation:

Cr₂O₇ ²⁻+6 Fe²⁺+7 H₂O→2Cr³⁺+6 Fe³⁺+14 OH   (I)

[0031] Although this equation proceeds rapidly at acidic pH (lower thanabout 4), it is not likely the dominant equation at neutral or alkalineconditions when chromate is the dominant species of hexavalent chromium.Because the reduction proceeds under highly acidic conditions, it may benecessary to titrate the pH of the soil back to neutral levels after thereduction is complete by using a base such as calcium oxide, calciumhydroxide, sodium hydroxide, sodium carbonate, or sodium bicarbonate,for example. The purely chemical reduction process is the preferredmethod for hexavalent chromium reduction when the contaminated site tobe treated is small. Depending on the particular site conditions, it maynot be feasible to utilize a purely chemical process.

[0032] In such cases, the detoxification may be accomplished via purelybiological or chemically assisted biological reduction. One embodimentof this invention thus involves the use of recyclable sludge from wastewater operations as a source for the reducing agent. As used herein, theterm “sludge” encompasses materials such as, but not limited to, returnactivated sludge (RAS), waste activated sludge (WAS), and leachate fromlandfills or from composting operations. In such cases, the reducingagents are biological instead of or in addition to chemical. The benefitto utilizing recyclable sludge is the extremely reduced cost of siteremediation. In particular, because the reducing agent may be usedmultiple times for enhancing soil flushing, if necessary, the amount ofoverdose of reducing agent may be significantly reduced (as discussedbelow).

[0033] In purely biological or chemically assisted biologicalreductions, the primary reducing agents are anaerobic or anoxicbacteria. When the cells are deprived of oxygen, they will be forced tofind an alternate source for growth. By choosing Cr⁶⁺ as an oxygensource, the chromium is simultaneously reduced to the trivalentoxidation state. Without wishing to be bound by theory, it has beenproposed that the biological reduction of Cr⁶⁺ to Cr³⁺ may be explainedby the exemplary redox reaction shown in Formula II. The cellularmaterial is generally represented by the formula (CH₂O), and thereduction of chromium from the chromate species results in the formationof trivalent chromium, hydroxide ions and carbon dioxide.

4 CrO₄ ²⁻+3CH₂O+7 H₂O→4 Cr³⁺+3CO₂+20 OH⁻  (II)

[0034] Although this method relies predominantly on bacterial reduction,a chemical reducing agent is also employed in a preferred embodiment inorder to hold the oxidation state and thus maintain an anaerobic oranoxic environment. The ideal pH for the chemically assisted biologicalreduction is preferably about 7, but it is also effective at slightlyalkaline pH levels above 7. Although the reduction will occur at acidicpH levels, the resulting trivalent chromium will remain in solution. Itis only under neutral or slightly alkaline pH that the chromium can be“fixed,” or precipitated out of solution as, for example, chromium oxideor chromium hydroxide. Because waste activated sludge is atapproximately neutral pH, it is itself a desirable reducing agentbecause it maintains a stable pH level.

[0035] The methods of the present invention involve initially samplingand analyzing representative portions of soil which require treatment.Several characteristics of the soil are preferably determined orcalculated to ensure effective treatment and remediation and todetermine the amount of reducing agent needed. Two such preferredcharacteristics are the measurement of the void space (the porosity) ofthe sample portion of unsaturated soil and the total volume of the soilto be treated. These measurements are useful for calculation of thevolume of water which is preferably employed to substantially andpreferably completely fill the void space. Specifically, the volume ofwater desired to fill the void space may be determined to beapproximately equal to the volume of soil multiplied by the percentageof void space. Typically, the porosity of the soil is about 30 volumepercent to about 50 volume percent, but may be greater or lower withoutaffecting significantly the operation of the invention.

[0036] It is preferred to also measure the concentrations of totalchromium and hexavalent chromium in the soil samples. The differencebetween these values is assumed for calculation purposes to be trivalentchromium. Using the hexavalent chromium concentration and the totalvolume of the soil to be treated, the theoretical reduction demand canbe calculated according to the dominant redox equation for theparticular reducing agent to be used. For example, the dominant redoxequation for ferrous species is Equation (I). As shown in Equation (I),for each mole of Cr(VI) to be treated, the theoretical reduction demandis three moles of ferrous sulfate.

[0037] The reaction amount of reducing agent needed for the in situreduction is calculated based on the theoretical reduction demand.Preferably, the reaction amount represents an overdose of about two toabout ten times the theoretical reduction demand, in order to provide asafety factor of design. Such a safety factor compensates foruncertainty of sampling and analysis, delivery inefficiency, and anyincomplete reactions to the extent such factors may have an effect. Aswill be described in further detail below, reduction may be accomplishedby a reducing agent which is either a chemical reducing agent alone, abiological reducing agent alone, or such a chemical reducing agent incombination with such a biological agent, depending on the reactionamount determined and the site conditions.

[0038] In a preferred embodiment of this invention, the pH level of thesoil is also measured from the sampled soil to determine if the pH levelof the soil is in an optimal range or if it would be preferable to addan activation agent to attain a preferred pH level for a desiredreaction rate. As explained previously, the ideal pH level forchemically assisted biological reduction is about 7, or slightly above.In contrast, the ideal pH level for a purely chemical reduction processis below about 7, and more preferably, below about 4. Based on the pHlevel of the soil, it may be desirable to calculate an amount ofactivation agent for raising or lowering the pH of the soil to thedesired level.

[0039] Preferably in the methods prior to treating with the reducingagent, the soil is prepared. This preparation preferably involvestilling approximately fifteen cm of the top soil using a rototiller orsimilar device and leveling the soil to produce a generally smoothand/or graded surface. Although a rototiller is used in a preferredembodiment, the tilling of the ground can be performed by any similarmachine known in the art, or by manual means. The mechanism or techniqueof leveling the soil or otherwise preparing the soil is not critical tothe methods and may be performed using any machine or device known inthe art. However, automated or mechanical methods are preferred for timeand efficiency reasons. Even if the surface of the soil is notcontaminated, preparation of the surface is preferred to allow for moreeven distribution of the reagents, which contributes to optimumefficiency and use of the reducing and activating agents.

[0040] At least one frame is built on a top surface of the prepared soilpreferably after preparing the soil as discussed above. A frame can begenerally any shape in transverse cross-sectional view, such asgenerally circular, oval, square, triangular, rectangular, or otherirregular shape. However, the frame preferably is generally square orrectangular with four sides for simplicity of construction. As can beseen more clearly by examining FIGS. 1 and 2A, a preferred frame isshown and generally referred to herein as frame 8, which has four sides10, 12, 14 and 16, which are preferably arranged to be perpendicular toone another and to the surface of the ground to create a generallyrectangular structure which extends vertically upwardly from the topsurface of the soil. The top 20 of the frame 8 and the bottom 18 of theframe are preferably open to the environment and soil to allow free flowof water for flushing. The bottom 18 preferably lies generally againstthe top surface 22 of the soil 30. The sides of the frame may beconstructed of any suitably rigid material capable of retaining thereducing agent and water required to effect the method. Examples of suchmaterials are metals, plastics, polymers and woods. In a preferredembodiment, the material for the frame is any type of wood, and it ismost preferred if the sides may be constructed of an inexpensive lumberor plywood. The sides of the frame may be any shape such as generallytubular or generally rectangular parallelepiped, but it is preferred ifthe sides are generally rectangular when viewed in the transverse crosssection of the side and when viewed in longitudinal cross section of theside. A frame may be constructed of multiple pieces or may comprise asingle piece. It is preferred that the frame is constructed of more thanone piece and more preferred if it is constructed of about four piecesfor ease in construction. If the frame is constructed of more than onepiece of material, the pieces may be connected to one another by anycommonly known method of joining materials, such as adhesives, welding,screws, nails or other hardware or connecting device. The particularmeans of joining the pieces of the frame may be determined depending onthe length of time during which the frame will be in use in a particularsituation.

[0041] The size of the frame depends on the size of the area and theterrain to be treated, but in a typical frame, each of the four sidesmeasures about 2 m to about 50 m when measured longitudinally. Morepreferably, when measured longitudinally, the sides measure about 5 m toabout 20 m. The opposite sides 12 and 16 of the preferred frame 8 aswell as sides 10 and 14 are preferably identical in length, but need notbe the same length as the adjacent side. The purpose of the frame is toserve as a biological and/or chemical reactor and to hold the requisiteamounts of reaction and activation reagent(s) and the water to beflushed through the soil.

[0042] The height 24 of the frame 8 may be calculated based on thereaction amount of reagent to be added. The depth of the reducing agentmay be calculated by dividing the total volume of the reducing andactivation agents (calculated as described above) by the surface area ofsoil to be treated. For example, if the surface area of the soil is 10m×10 m (100 m²) and 15 m³ of reducing agent (without additionalactivation agent) are needed, the estimated depth of the reagent wouldbe (15 m³/100 m²)=0.15 m (15 cm). The height of the frame shouldpreferably be somewhat greater than the height of the reducing andactivation agents to accommodate additional volumes of water. In apreferred embodiment, the frame is at least about 30% to about 60%higher and preferably about 10 cm to about 20 cm higher than thecalculated height of the reducing and activation agents. For example, ifthe calculated depth of reducing and activation agents is 10 cm, a framebetween about 20 cm and about 30 cm high would be preferred.

[0043] In one embodiment, as shown in FIG. 2A, at least one recoverywell 46 is installed on the down gradient side 44 of the frame. By “downgradient” is meant the lowest level of water flow. In FIG. 2A, thedirection of water flow is indicated by a solid arrow. At least one pump48 or other circulating means is installed at the bottom of the recoverywell 46. As will be explained in further detail below, the combinationof the pump and the recovery well allows water to be pumped back up intothe frame and recirculated. The direction of pumped water flow isdesignated by the dotted arrows in FIG. 2A.

[0044] The chemical reducing agent used in the methods may be in aliquid or a solid form; solid reducing agents may be either in a powderor a granular form. In some cases, the reducing agent may be commonlyobtained in solution, such as, for example, waste pickle liquor fromsteel production or metal finishing and molasses. In such cases, a stocksolution may be prepared by dissolving the reducing agent in solution ata desired pH level, depending on the particular reducing agent, so thatthe reducing agent is not reactive in air to avoid loss of potency. In apreferred embodiment, the reducing agent is a solid and thus noadditional preparation before use is necessary. The use of a solidreducing agent thus eliminates the necessity of preparing andsubsequently diluting a stock solution, thereby reducing cost and timerequired for such a step and making the process further economical.

[0045] As discussed above, it may be desirable when performing a purelychemical reduction to add an activation agent to adjust the pH of thesoil to a preferred level. Since the reaction between Cr(VI) and ferroussulfate proceeds faster at lower pH levels, if the soil pH is greaterthan about 7, it may be desirable to decrease the pH by adding an acidicactivation agent. Preferably, if the pH of the soil is greater thanabout 4, it may be desirable to decrease the pH by adding an acidicactivation agent. In some areas, the pH level of contaminated soil mayalready be below about 7, and preferably below about 4, and thus anactivation agent would not be necessary for performing a purely chemicalreduction. If an activation agent is added to reduce the pH of the soilto optimal acidic level, it may be necessary to titrate the soil back toneutral levels upon completion of the reduction by adding a base such ascalcium oxide, calcium hydroxide, sodium hydroxide, sodium carbonate, orsodium bicarbonate, for example.

[0046] In some situations, the site conditions make purely biological orchemically assisted biological reduction more feasible methods. Forexample, the roasting of chromite ore (Cr(OH)₃ or Cr₂O₃) under alkalineoxidizing conditions (typically with lime) results in contaminatedresidues which are highly alkaline due to the use of lime in theroasting process. Hence, typically the tailings in these areas arebuffered to the pH of lime, normally about 8.5 to about 9.0. Althoughthe ideal pH for chemically assisted biological reduction is about 7,the reaction will proceed under alkaline conditions as well, and becausethe pH of the soil is already buffered to alkaline level, a biologicalreduction would be most practical. In such circumstances, it ispreferred to use chemically assisted biological reduction rather thanpurely biological reduction because the chemical reducing agent holdsthe oxidation state and maintains an anaerobic or anoxic environment.

[0047] If the activation agent is a solid (in either powder or finegranular form), no additional preparation is necessary prior to use.However, if the activation agent is to be added in solution form, astock solution may be prepared by dissolving the activation agent at aconcentration which would achieve the desired pH level in situ. An idealpH is lower than about 7, but it is preferred if the pH level is lowerthan about 4.

[0048] The activation agents are preferably pH adjusting chemicals(acids or bases) and aqueous solutions of such acids or bases. Commonlyused acidic activation reagents include sulfuric acid, hydrochloricacid, nitric acid, and acetic acid. Although organic acids may also beused, inorganic acids are preferred. Commonly used basic activationagents include calcium oxide (lime), calcium hydroxide (hydrated lime),sodium hydroxide (caustic soda), sodium carbonate (soda ash), and sodiumbicarbonate (baking soda). These materials are listed as exemplary acidsand bases and are not intended to be limiting.

[0049] The biological agents which may be used in the methods include,but are not limited to, sludge from wastewater, waste activated sludge(WAS) and compost from municipal or from industrial wastewater and solidwaste operations. Preferably, the biological agent is waste activatedsludge, which is mixed culture from municipal wastewater treatmentplants, operating with selective zone biological nutrient removal (BNR).Such a selective process results in a high population of anoxic andanaerobic bacteria. However, any sludge should have enough mixed cultureto be effective as a reducing agent. Although return activated sludge(RAS) may also be utilized, it is typically dilute and contains onlyabout 1 to about 3% solids. In contrast, WAS typically contains about 25to about 30% solids. Therefore, if the area to be treated is in alocation such that the reducing agent must be transported anysignificant distance, it is preferable to utilize the more concentratedWAS.

[0050] Before introducing the reducing agent and activation agent, ifany, to the soil, it is preferred that the soil is wet with water topromote better distribution of the reagent(s) through the unsaturatedsoils. The amount of water to be added is an estimated overdose of thepreviously calculated total void volume. In a preferred embodiment,water completely fills the void space of the soil. In one preferredembodiment of the invention, a surfactant may be optionally added withthe water to enhance the wetting process when a purely chemical processis to be utilized. Suitable surfactants include, without limitation,non-ionic surfactants such as alcohol ethoxylates, alkyl phenolethoxylates, and lauryl alcohols. Although not preferred, ionicsurfactants are also within the scope of this invention. The wettingfurther opens and widens any pores in the soil, thus allowing thereagents to permeate, and enhancing the effectiveness of the method.Therefore, it is preferred than any surfactant selected be capable offacilitating wetting of the soil without having a significant effect onthe reaction.

[0051] The wetting may be accomplished by any method known in the artfor delivering water to soil, such as, but not limited to, hoses and/orsprinkler systems, which may be mounted on the frame, on poles, ontrucks or on similar structures. In a preferred embodiment, the watermay be delivered by perforated slow-drip hoses which are arranged aboveor on the top surface of the soil to promote even water coverage. Asshown in FIGS. 2A and 2B, the hose 38 is preferably arranged evenly overthe top surface 22 of the soil 30 and connected to a water source 36 forexample by a hose 40. The size of the perforated holes 42 anddistribution of the perforated holes is not critical, as long as theyare sufficiently spaced and large enough to allow for even delivery ofthe water to the soil. After wetting the soil, the hose 38 is preferablyremoved from the upper surface of the soil.

[0052] If a surfactant is to be used to enhance the wetting process, thesurfactant is preferably dissolved in water to make a solution. Such asolution is evenly distributed over the top surface of the soil,preferably by manual spraying or by means of an in-line garden hosesprayer, for example. Other means of applying the surfactant to the soilare also within the scope of the invention.

[0053] The activation agent, if added, and the reducing agent arepreferably applied to the top or upper surface of the soil within theframe. If the reagent is in solution form, it is preferably applied tothe upper surface of the soil using a known irrigation device typicallyused for delivering chemicals for lawn care. Examples of such devicesare a sprinkler system, a hose, a manual spray bottle, and the like. Ina preferred embodiment, the reagent is in a solid form and it is spreadgenerally evenly in the frame on the top surface of the soil. The use ofa solid reducing agent thus eliminates the necessity of preparing anddiluting a stock solution and applying a liquid to the soil.

[0054] If the solid reagent is a granulate with a particle size greaterthan about 3 cm in diameter, it is preferably crushed prior toapplication to obtain a particle size of preferably less than about 5 mmin diameter, and more preferably less than about 1 mm in diameter,wherein diameter is defined as above. It is preferred that the solidreagents are purchased pre-screened to a desirable size. In such a casea mine or manufacturer of the particular reagent may utilize a pug milland a screening machine to crush larger particles to a desirable size.Alternatively, a backhoe, front-loader, pulverizer, or any other similardevice known in the art may be used for crushing the particles. Crushingmay also be accomplished by manual means. Subsequently, a screeningmachine or other similar device may be used to sieve the crushedparticles to a desirable size.

[0055] If used in solid form, the activation and reducing agents may bespread on the soil in the frames by any method known in the art forspreading solids on soil. These may include, but are not limited to,automated spreaders which are connected to the top of the frame andwhich provide even distribution of the reagents, mechanical spreadersand manual spreaders. A spreader 50 is shown in FIG. 2A, whichillustrates the solid reducing and/or activation agents being applied tothe top surface 22 of the soil 30 in the frame 8. In a preferredembodiment, the reducing agent is loaded onto a truck and dumped ontothe top surface of the soil in the frame. Leveling the reagent(s) isgenerally accomplished by any known method, such as by using anautomated truck or other machine or by manual means. In a preferredembodiment, the reducing and activation agents are generally leveledwith rakes.

[0056] Once the agents have been applied to the soil, the soil isflushed to dissolve the reducing agent and activation agent, if present.The soil may be flushed with water or an acceptable aqueous solution atsuch a rate as to dissolve the reducing agent and to generally allow allof the reagent(s) to percolate through the soil in about one to about afew days. Any water may be used, including tap water, de-ionized water,distilled water, or mixtures, but tap water is preferred for simplicityand low cost. As discussed previously, the water delivery may beaccomplished by any method known in the art. It is preferred thatperforated slow drip hoses be arranged above or on the top surface ofthe reagent(s) in the frame and connected to a water source 36 topromote generally even coverage of the soil to be treated. If thereducing and/or activation agent has been added in solution form, thehose may be laid out directly on the top surface of the soil within theframe. The rate of water delivery may vary depending on whether thereducing agent and activation agent are in solid or liquid form and thesolubility of the reducing and activation agents. Since lowerpermeability soils may not allow water (and therefore the reagents) topercolate through the soil effectively, these procedures are generallyapplicable to contaminated soil with permeability greater than about1×10⁻⁵ cm/sec, and preferably greater than about 1×10⁻⁴ cm/sec.

[0057] Upon reacting with the reducing agent, the hexavalent chromium isreduced to the trivalent form, which is much less soluble thanhexavalent chromium. Immediately after reduction, trivalent chromiumwill generally be in the form of aqueous trivalent chromium ions[Cr(H₂O)₆ ³⁺]. In time and under neutral or basic pH conditions, theaqueous trivalent chromium, whether above or below groundwater level,will be transformed to chromic hydroxide (Cr₂O₃.nH₂O), chromite(FeCr₂O₄), or other stable trivalent chromium minerals. These trivalentchromium minerals are generally not soluble unless either oxidized ortreated with acid, and thus represent a lesser health concern thanCr(VI).

[0058] In one embodiment of this invention, only a fraction of thereducing agent may be applied in one application. Repeated applicationsmay be performed until the total amount of reducing agent has beendelivered over about one to about a few days. The need for repeatedapplication may be determined by factors including the permeability, thedepth, and the degree of contamination of the soil to be treated. Forexample, treatment of more than one day may not be necessary if the soilpermeability is greater than about 1×10⁻⁴ cm/sec with less than about 10m depth and/or hexavalent chromium concentration generally not exceedingabout 1,000 mg/Kg.

[0059] In a preferred embodiment of this invention, the flushed soil maybe re-sampled and re-analyzed to verify the effectiveness of the methodbased on the amount of hexavalent chromium present in the flushed soilafter addition of the reducing agent and flushing with water. The methodmay be repeated using a similar or a stronger reducing agent if it isdetermined to be necessary or desired based on the amount of Cr(VI) inthe soil after the initial treatment. If the amount of hexavalentchromium is not sufficiently reduced, the method may be repeated untilthe chromium concentration in the soil is at a desired level, which ispreferably no greater than the TCLP limit of 5 mg/L.

[0060] In one preferred method of the invention, following the stepsdescribed above for re-sampling the soil and determining that theconcentration of Cr(VI) is not sufficiently reduced, repeating thetreatment of the soil may involve the installation of recovery wellslocated on the down gradient side of the frame. Pumps or othercirculating means are installed beneath the ground so that ground watercomprising the unused portion of the reducing agent or unreactedhexavalent chromium solution can be pumped up from the recovery wellsand applied to the soil in the frame, thereby affecting a secondtreatment. When the method for detoxification is purely biological orchemically assisted biological reduction, or when the groundwater is tobe treated as well as the soil, as discussed below, it is preferable ifthe recovery wells are installed so as to allow for recirculation.Following the flushing of the soil, the soil may be sampled to determinethe effectiveness of the treatment based on the amount of hexavalentchromium in the treated soil. If the amount of Cr(V) is not sufficientlyreduced, the water is preferably recirculated repeatedly until a desiredconcentration of hexavalent chromium is achieved.

[0061] Following treatment of the contaminated area using a purelychemical process, the pH of the soil is preferably readjusted to adesired level if necessary. For example, if the treatment is performedat a pH level of about 4, lime, or any other basic chemical known in theart as previously described, may be added to the soil to raise the pH toa level of about 7. Additionally, it may be desirable followingtreatment of the soil to redevelop the area, such as by planting grassor trees, for example.

[0062] A method according to this invention allows for the in situreduction of hexavalent chromium to trivalent chromium in groundwatercomprising hexavalent chromium. In many contaminated sites, the totalchromium concentration in the groundwater is greater than the maximumcontamination level (MCL) of 50 ppb, and thus detoxification of thegroundwater is required. Sampling of the groundwater to determine areaction amount of a reducing agent and preferably to determine theconcentration of hexavalent chromium may be accomplished, for example,by drilling a hole into the soil and installing permanent wells or byutilizing a direct push process, in which the soil is pushed down with asampler in order to obtain a water sample in-situ without the need fordrilling or pumping. The sample of ground water may be analyzed todetermine the concentration of chromium by any method well known in theart. For example, methods of analysis for groundwater are described inEPA SW846 methods 7195, 7196, 7197, and 7198, each of which isincorporated herein by reference for this purpose. The preferredmethods, which are the most sensitive, are 7197 and 7195.

[0063] Once the groundwater has been treated to determine the reactionamount of reducing agent, the method for reduction of the hexavalentchromium in the groundwater preferably includes steps as describedpreviously, including providing a frame on a top surface of the soil,providing a recovery well and a circulating means on a down gradientside of the frame, and wetting the soil. The reducing agent (andactivation agent if desired) is spread on the top surface of the soil,and the soil is flushed with water to dissolve the reducing agent and toallow the reducing agent to preferably percolate through the soil toreach the groundwater. The water comprising the reducing agent may thenbe pumped from the recovery well below the soil and applied to the topsurface of the soil. In one embodiment, the groundwater may be sampledafter treatment to determine the effectiveness of the treatment based onan amount of hexavalent chromium present in the groundwater. If theamount of hexavalent chromium concentration is not sufficiently reduced,the steps of the method including wetting the soil, spreading a reducingagent, flushing the soil, and pumping the water from the recovery well,may be repeated, if desired, until the concentration of C(VI) in thegroundwater achieves a desired level, such as no greater than 50 ppb inone embodiment.

[0064] One embodiment of this invention is for the reduction ofhexavalent chromium to trivalent chromium in soil comprising hexavalentchromium in a hydraulically isolated area, such as with, but not limitedto, slurry walls or sheet piling. Many contaminated areas, such as thosecontaining contaminated waste products such as RAS and WAS, have beenpurposefully hydraulically isolated as a means for preventing thecontaminants from leaching out into the air, groundwater and adjoiningsoil areas, rather than attempting to treat the contamination. In suchareas, it may be desirable to utilize a recycling and recirculatingmethod in order to detoxify the soil and reduce Cr(IV) to Cr(III). Insuch situations, chemically assisted biological reduction is thepreferred method of treatment, as described above, although purelybiological reduction would also be possible in such areas. As previouslydescribed, such a method involves sampling the soil, providing a frameon a top surface of the soil, applying a reaction amount of a reducingagent, and flushing the soil with water. After an initial reactionamount of chemical reducing agent has been percolated through the soil,the waste water, which contains unreacted chemical reducing agent, aswell as biological reducing agent, may be pumped back from an aquiferbelow the soil and reapplied to the top surface of the soil via theframe.

[0065] Under some circumstances, it may be desirable to perform thereduction of hexavalent chromium as a subsurface operation rather thanin frames on the top surface of the soil. Particularly, since biologicalreduction is an anaerobic process which releases hydrogen sulfide gas(having a characteristic rotten-egg odor), such a subsurface operationmay be preferred if the soil to be remediated is in or near to apopulated area. Under such circumstances, if the contaminated soil isfound below the surface (and the top layer of soil may thus be free ofhexavalent chromium), the method may comprise removing a top layer ofsoil which is free of Cr(VI) (about 30 cm, for example) from the area,thereby creating an artificial depression. The soil is sampled, asdescribed previously, and the void space and volume of soil to betreated are determined, as well as the concentration of hexavalentchromium in the contaminated soil and the pH of the soil. As previouslydescribed, the reaction amount of reducing agent may be calculated, andthe depth of reducing agent (and activation agent if necessary) may becalculated based on the reaction amount of reduction and activationagents and the surface area of the soil in the artificial depression.

[0066] Following the wetting of the soil, as previously described, thecalculated amount of chemical reducing agent and BNR sludge or otherbiological reducing agent (and activation agent, if desired) may bespread directly in the depression and flushed with water, without theneed for providing a frame to contain the reducing agent. The layer oftop soil which was removed may then be replaced over the top surface ofthe soil, and a recycling/recirculating method as previously describedmay be performed, if desired. Alternatively, if the top layer of soil iscontaminated, a layer of clean soil free of hexavalent chromium (such asto a depth of about 30 cm) may be provided on top of the surface of thecontaminated soil after applying the reducing agent and flushing withwater. In both cases, the presence of a clean layer of soil that is freeof hexavalent chromium and the microbial activities which occur thereinwill act as a biofilter to help remove odor-causing byproducts which mayarise, and will make the remediation site less distasteful. Such a toplayer may be seeded with grass or planted with trees, if desired.

[0067] The methods of site remediation via recirculation, either byinstallation of recovery wells or in a hydraulically isolated area, arehighly advantageous. The results are greatly enhanced, which makesreduction of high concentrations of Cr(VI) to very low levels easilyattainable. Furthermore, the costs of remediation are significantlyreduced as the amounts of reducing agent which are needed are muchlower. In highly contaminated sites, in which enormous quantities ofreducing agent would be needed to provide an overdose of two to tentimes the theoretical reduction demand, recirculation allows for the useof only a slight overdose of reducing agent.

[0068] Under some circumstances, it may be desirable to perform a purelybiological reduction. Specifically, in some hydraulically isolatedcontaminated areas or in areas in which recovery wells have beenconstructed, it may not be necessary to add an initial amount ofchemical reducing agent if the available biological reducing agent, suchas return activated sludge, waste activated sludge, or leachate fromgroundfill or composting operations contains sufficient reducing agent.Typically, these waste products generally contain very strong mixedorganic reducing agents. Under such circumstances, it may be determinedupon sampling the soil, as described above, that the waste product hassufficient theoretical reduction power. In other words, it may becalculated by estimating the chemical oxygen demand (COD) of the wasteproduct that a particular depth of the waste product may be able toachieve an overdose of the theoretical reduction demand. Accordingly,after a frame is provided on a top surface of the soil, as previouslydescribed, the soil may be wet and the particular depth of the wasteproduct, as determined above, may be pumped from an aquifer below thesoil, applied to the soil, and distributed into the frame. Additionalwater may then be added for about one day to about several days at anyrate which is deemed appropriate based on the particular conditions toslowly dissolve the sludge and deliver it to the contaminated soil. Theadvantage to using such waste products as reducing agents is low cost:because they already contain reducing agent, there is no need to add anyadditional chemical or biological reducing agent to the soil duringtreatment.

[0069] There are several significant advantages to the methods of thepresent invention. Firstly, the methods utilize inexpensive, readilyavailable materials, such as simple lumber may be used for building theframe, standard hoses may be used for applying the water, and theapplication of inexpensive reducing agents, such as ferrous sulfate andbiological agents from beneficial use of municipal wastewater ortreatment wastes. Furthermore, the manner in which the soil is preparedfor treatment is straightforward and easy to accomplish withoutincurring significant expenses, such as those required when buildingreactors for ex-situ soil treatment. This invention further providesmethods for the in-situ detoxification of soil and groundwater byreducing the hexavalent chromium without excavation or the need formechanical mixing. The use of the frame provides a way to dissolve bulkgranular reagents for direct delivery by gravity without the need toprepare solutions. Finally, the use of surfactants to wet the soil andreduce the surface tension provides a better and more uniform contactbetween the reagents and contaminants. Such methods fulfill a need forsafe, controllable, easy methods of chromium reduction which can beaccomplished on a large scale and which are environmentallyadvantageous. The advantages outlined above make the methods feasible onlarge contaminated sites without significant costs. The methods areeffective in reducing the amount of hexavalent chromium to extremely lowlevels. Finally, the methods allow for the use of return activatedsludge or wastewater as a source for the reducing agent, which furtherreduces the cost of soil remediation. This is especially important inhighly contaminated areas as it further serves to reduce the cost oftreatment.

[0070] In addition to providing methods for the in-situ reduction ofCr(VI), this invention also provides a system for the reduction ofhexavalent chromium in contaminated soil. As shown in FIGS. 1 and 2A anddescribed previously, the system includes a frame 8 open at a top 20 anda bottom 18 configured to retain water and a reaction amount of areducing agent capable of reducing hexavalent chromium to trivalentchromium, as described previously. As shown in FIG. 2A and describedpreviously, the system further includes a hose 38 positioned withrespect to the frame 8 such that water can be provided through the topand bottom of the frame 8, a spreader 50 capable of delivering areaction amount of a reducing agent to the frame, and a water source 36.Such an inexpensive system containing readily available components isuseful for the in situ reduction of Cr(VI) to Cr(III).

[0071] This invention will now be described in more detail with respectto the following non-limiting examples:

EXAMPLE 1

[0072] Bench-scale tests were used to evaluate the viability of usingdomestic wastewater treatment biological nutrient removal (BNR) sludgeand a chemical reducing agent (ferrous sulfate) to reduce hexavalentchromium to trivalent chromium. For these tests, soil samples werecollected from the Patapsco Waste Water Treatment Plant (WWTP). The soilsamples were sieved and rinsed, and debris, including pebbles, brickpieces, metal, wood, etc., was removed. The soil samples were thenspiked with potassium dichromate and about 300 mL of the spiked soilsamples were packed into 500 mL disposable filtration units (Cole PalmerCatalog No. A-06730-54). Two levels of spiked potassium dichromate soilswere used: a higher level (“Spiked High”) containing about 450 mg/Kg ofhexavalent chromium (or about 300 mg in each filtration unit), and alower level (“Spiked Low”) containing about 150 mg/Kg of hexavalentchromium (or about 100 mg in each filtration unit).

[0073] Following the application of the appropriate reducing agent, asdescribed below, tap water (200 mL) was poured on top of each filtrationsystem. After the water had completely flowed through the system, theeffluent water was poured back on the top portion of the system tosimulate a pump. This process was repeated throughout the day withsingle run times ranging from five minutes to six hours. Except for afew slow flowing systems, test runs were paused during lunch hours,after office hours, and during weekends. Vacuums were created within thesystem using latex gloves to prevent water flow, thereby pausing thesystem. During weekends, the systems were run for approximately twohours on both Saturday and Sunday. Conditions were run for a minimum ofeight days. Effluent samples, each 5 mL, were typically taken on thefirst, second, fourth, and eighth days of the test runs. At the end ofthe runs, the soil samples were taken out of the filtration unit andanalyzed as follows. Each 5 mL sample was diluted with 100 mL of waterand the diluted samples were tested for hexavalent chromium andoccasionally, total chromium.

[0074] Three different remediation approaches were taken in order toconvert the hexavalent chromium to trivalent chromium: (1) using purelychemical reduction; (2) using purely biological reduction; and using acombination of chemical and biological reductions.

[0075] Chemical Reduction Method

[0076] The chemical reduction method involved adding dry ferrous oxide(about 8 g) directly into the contaminated soil and stirring it in tosimulate a tiller. The chemical reaction between the potassiumdichromate in the soil and the added ferrous sulfate resulted in a dropin the pH of the system. Two conditions were tested, one using onlyferrous sulfate and a second in which lime was included to maintain a pHlevel above 8. The results of the chemical reduction are shown in Table1 below: TABLE I Cr (VI) (mg/L) in Filtrate pH Test Condition Day OneDay Nine Day Nine Control, Spiked High 956 1,008 7 FeSO₄ alone, SpikedHigh N.D. N.D. 4 FeSO₄ w. Lime, Spiked High 1,260 1.1 12.5 Control,Spiked Low 210 250 7 FeSO₄ alone, Spiked Low N.D. — 4

[0077] Biological Reduction

[0078] The second remedial method, using purely biological reduction,involved adding about 60 g of BNR sludge cakes, collected from BackRiver WWTP of Baltimore, Md., on top of the contaminated soil. Threedifferent conditions were tested in this method: (1) sludge alone; (2)sludge and a carbon food source; and (3) a combination of sludge, acarbon food source, and lime (to maintain pH above 7). Although it wasfound that the sludge alone had the ability to convert hexavalentchromium to trivalent chromium, the reducing rate was slow. Accordingly,to increase the remediation rate, a carbon food source (one tablespoonor about 12 mL of syrup) was added and mixed into the sludge. During thebiological activity, the pH of the system dropped from an ideal 7 to anacidic 4.5. To counter this change, seven tablespoons (about 80 mL) oflime were added and mixed into the sludge, thus raising the pH back to7. The results of biological reduction under the three conditions areshown in Table 2 below: TABLE 2 Cr (VI) (mg/L) in Filtrate pH TestCondition Day One Day Nine Day Nine Control, Spiked High 956 1,008 7Sludge alone, Spiked High 735 420 7.5 Control, Spiked Low 210 250 7Sludge w. syrup, Spiked Low N.D. N.D. 4.5

[0079] Combination of Chemical and Biological Reduction

[0080] The third remedial method, a combination of chemical andbiological reduction, utilized both ferrous oxide and BNR sludge toconvert hexavalent chromium to trivalent chromium. In this method,components of the chemical and biological reduction methods, aspreviously described, were combined. The results of the combinedchemical and biological reduction are shown in Table 3 below: TABLE 3 Cr(VI) (mg/L) in Filtrate pH Test Condition Day One Day Nine Day NineControl, Spiked High 956 1,008 7 Sludge & FeSO₄, Spiked High 189 147 6Sludge, syrup, lime, & FeSO₄, 14 27 6.5 Spiked High Control, Spiked Low210 250 7 Sludge & FeSO₄, Spiked Low N.D. N.D. 4.5 Sludge, syrup, &FeSO₄, N.D. N.D. 4.5 Spiked Low

[0081] From these results, it can be concluded that the best results ofhexavalent chromium detoxification can be obtained using chemicallyassisted biological reduction. Although purely chemical (ferroussulfate) treatment does reduce the hexavalent chromium in soils, suchtreatment often results in pH levels in an undesirable range (either toolow at about 4 or too high at above 12). Additionally, based on theseresults, the upper contamination level of hexavalent chromiumconcentration in soils that can be reliably treated by the proposedmethod to achieve the stringent levels that meet the newly proposedlevel by the state of California appears to be about 150 to 450 mg/Kg.

EXAMPLE 2

[0082] A chromium tailing pile at the Pautuxent Waste Water TreatmentPlant (WWTP) in Baltimore, Md. measuring approximately 100 m in length,50 m in width, and 14 m in depth is required by the State of Maryland tobe remediated to the standard of 5 mg/L chromium in TCLP. More than 200samples have been taken from more than 60 borings and analyzed. Thetotal chromium concentrations range from non-detect to 29,400 mg/Kg, andhexavalent chromium concentrations range from non-detect to 6,780 mg/Kgwith the average estimated to be 200 mg/Kg. The EP Toxicity (a similarbut early version of TCLP) chromium concentrations range from non-detectto 97 mg/L. The pH of the tailing ranges from 6.3 to 7.9, with theaverage being 6.9, and the average alkalinity exceeds 10,000 mg/Kg(mainly present as CaCO₃), so that it would be economically prohibitiveto lower the soil pH with bulk acid. The specific gravity of the soil is2.3.

[0083] The selected method of treatment for the pile is reduction withthe waste activated sludge (WAS) available at the WWTP itself. Thetheoretical reduction power of the WAS is estimated using its chemicaloxygen demand (COD), which is about 8,000 mg/L of O₂ (0.25 molesoxygen/L, or 1.0 equivalents/L). The WAS is estimated to be about 3% insolids and about 1.0 in density. The theoretical reduction demand of thecontaminated tailing pile is estimated to be 6.087×10⁶ g (117,000 molesor 351,000 equivalents to be reduced from hexavalent to trivalentchromium) as chromium and the reaction amount, accounting for aconservative overdose of 10 times, is calculated to be 3,510,000 L ofWAS. Therefore, the depth of the WAS is preferably about 70 cm, and theheight of the frame is desirably at least one meter.

[0084] The pile is prepared by mechanically (using a bulldozer, forexample) leveling the top of the pile after clearing the trees andvegetation. A series of 50 square frames measuring 10 m in length, 10 min width, and 1 m in depth are constructed of plywood such that eachadjoins the next and such that the whole pile is covered. Walkways areconstructed with plywood on top of the frames for maintenance. Gardensoakers (e.g., U.S. Pat. No. 5,368,235) or hoses with small holes (lessthan 1 mm diameter evenly punched along the entire length of the hose)are arranged on the top surface inside of the frames in such a way thatthey are evenly distributed. The soakers or hoses are connected to awater supply and water flows at about 1 L/minute-m² for at least 24hours to saturate the tailings. The water is turned off and WasteActivated Sludge from the WWTP is pumped and distributed into the framesso that each one is filled at about 70% of the depth with WAS. Water isturned back on at a rate to approximately maintain the head of thesludge inside the frames for seven days and then turned off. The sludgeis allowed to sit until dried. Random core samples of treated tailingsare taken with a Geoprobe and analyzed for hexavalent chromiumconcentrations. If any of the hexavalent chromium concentrations arefound to exceed 80 mg/Kg, additional WAS is placed in the framed areascontaining concentrations exceeding the desired limit. If all thehexavalent chromium concentrations are found to be within 80 mg/Kg,confirmation TCLP samples are taken to provide evidence that the cleanupgoal has been reached. After the successful remediation has beendemonstrated, the frames are removed, and the area is redeveloped orseeded with grass.

[0085] It will be appreciated by those skilled in the art that changescould be made to the embodiments described above without departing fromthe broad inventive concept thereof. It is understood, therefore, thatthis invention is not limited to the particular embodiments disclosed,but it is intended to cover modifications within the spirit and scope ofthe present invention as defined by the appended claims.

1-40. (Canceled)
 41. A method for the in-situ reduction of hexavalentchromium to trivalent chromium in soil comprising hexavalent chromium,comprising: (a) sampling soil comprising hexavalent chromium todetermine a reaction amount of a reducing agent; (b) wetting the soil;(c) spreading a reducing agent on a top surface of the soil; (d)flushing the soil with water to dissolve the reducing agent; and (e)covering the top surface of the soil with a layer of clean soil free ofhexavalent chromium.
 42. The method according to claim 41, wherein thesoil comprising hexavalent chromium to be reduced comprises a top layerwhich is free of hexavalent chromium, and the method further comprisesremoving the top layer of the soil which is free of hexavalent chromiumbefore step (b); and step (e) further comprises using the removed toplayer of the soil as the layer of clean soil in step (e).
 43. A systemfor the in-situ reduction of hexavalent chromium in soil comprisinghexavalent chromium, comprising: (a) a frame open at a top and a bottomconfigured to retain water and a reaction amount of a reducing agentcapable of reducing hexavalent chromium to trivalent chromium; (b) ahose positioned with respect to the frame such that water can beprovided through the top and bottom of the frame; (c) a spreader capableof delivering a reaction amount of a reducing agent to the frame; and(d) a water source.
 44. The method according to claim 41, wherein step(a) further comprises: (i) obtaining a soil sample; (ii) determining anamount of hexavalent chromium in the soil sample; (ii) calculating atheoretical reduction demand; and (iii) determining the reaction amountof reduction agent to be used based on the theoretical reduction demand.45. The method according to claim 41, wherein step (a) further comprisesmeasuring a pH of the sampled soil.
 46. The method according to claim41, further comprising: (f) sampling the flushed soil to determine theeffectiveness of the treatment based on an amount of hexavalent chromiumpresent in the flushed soil.
 47. The method according to claim 46,further comprising repeating steps (b)-(d) if the amount of hexavalentchromium concentration is not sufficiently reduced.
 48. The methodaccording to claim 41, further comprising: (g) providing a recovery wellon a down gradient side of the soil; and (h) pumping water comprisingthe reducing agent from the recovery well below the soil and applyingthe pumped water to the soil.
 49. The method according to claim 48,wherein the flushed soil after pumping has an amount of hexavalentchromium no greater than 5 mg/L.
 50. The method according to claim 41,wherein the reducing agent is selected from the group consisting offerrous salts, sulfide salts, sodium thiosulfate, sodium benzoate, andorganic reducing agents.
 51. The method according to claim 50, whereinthe reducing agent is a ferrous salt selected from the group consistingof ferrous sulfate, ferrous chloride, and ferrous sulfite.
 52. Themethod according to claim 50, wherein the reducing agent is a sulfidesalt selected from the group consisting of sodium sulfide and potassiumsulfide.
 53. The method according to claim 50, wherein the reducingagent is an organic reducing agent selected from the group consisting ofcitric acid, methanol, ethanol, and formaldehyde.
 54. The methodaccording to claim 41, wherein the reducing agent is in solution. 55.The method according to claim 41, wherein the reducing agent is one of apowder or a fine granulate.
 56. The method according to claim 55,further comprising dissolving the reducing agent in solution prior tospreading the reducing agent on top of the soil.
 57. The methodaccording to claim 41, wherein the method further comprises calculatinga void space of the soil before wetting the soil in step (b) and step(b) further comprises adding a quantity of water sufficient to fill thevoid spaces.
 58. The method according to claim 41, wherein step (b)further comprises applying a surfactant to the soil.
 59. The methodaccording to claim 58, wherein the surfactant is a non-ionic surfactantselected from the group consisting of ethoxylates, alkyl phenolethoxylates, and lauryl alcohols.
 60. The method according to claim 41,wherein step (c) further comprises adding an activation agent with thereducing agent to adjust a pH of the soil.
 61. The method according toclaim 60, wherein the activation agent is one of a solution, a powder,or a fine granulate.
 62. The method according to claim 60, wherein theactivation agent is an acid selected from the group consisting ofsulfuric acid, hydrochloric acid, nitric acid, and acetic acid.
 63. Themethod according to claim 60, wherein the activation agent is a baseselected from the group consisting of calcium oxide, calcium hydroxide,sodium hydroxide, sodium carbonate, and sodium bicarbonate.
 64. Themethod according to claim 41, wherein the reducing agent is a biologicalagent.
 65. The method according to claim 64, wherein the biologicalagent is sludge from wastewater.
 66. The method according to claim 64,wherein the biological agent is return activated sludge.
 67. The methodaccording to claim 64, wherein the biological agent is leachate fromlandfill or from composting operations.
 68. The method according toclaim 64, wherein the biological agent is waste activated sludge. 69.The method according to claim 64, wherein the biological agent iscompost material from municipal wastewater, industrial wastewater, orsolid waste operations.
 70. The method according to claim 41, whereinthe soil treated by the method has an amount of hexavalent chromium nogreater than 5 mg/L.
 71. The system according to claim 43, wherein thereducing agent is selected from the group consisting of ferrous salts,sulfide salts, sodium thiosulfate, sodium benzoate, and organic reducingagents.
 72. The system according to claim 71, wherein the reducing agentis a ferrous salt selected from the group consisting of ferrous sulfate,ferrous chloride, and ferrous sulfite.
 73. The system according to claim71, wherein the reducing agent is a sulfide salt selected from the groupconsisting of sodium sulfide and potassium sulfide.
 74. The systemaccording to claim 71, wherein the reducing agent is an organic reducingagent selected from the group consisting of citric acid, methanol,ethanol, and formaldehyde.
 75. The system according to claim 43, whereinthe reducing agent is a biological agent.
 76. The system according toclaim 75, wherein the biological agent is sludge from wastewater. 77.The system according to claim 75, wherein the biological agent is returnactivated sludge.
 78. The system according to claim 75, wherein thebiological agent is leachate from landfill or from compostingoperations.
 79. The system according to claim 75, wherein the biologicalagent is waste activated sludge.
 80. The system according to claim 75,wherein the biological agent is compost material from municipalwastewater, industrial wastewater, or solid waste operations.